A 3-way catalytic converter can only convert pollutants in an optimal manner if the fuel/air ratio is within a narrow range around lambda≈1. Said range is referred to also as the catalytic-converter window. Only with fuel/air ratios of said kind will the exhaust composition be such that the oxygen released when the nitrogen oxides are reduced will suffice to almost completely oxidize the exhaust gas's HC and CO components into CO2 and H2O. Mixing is therefore controlled in an internal combustion engine having a 3-way catalytic converter by what is termed a lambda controller to a target value of lambda≈1. To compensate brief fluctuations in the fuel/air ratio the catalytic converter also contains a coating (washcoat) made of a material, for example Ce2O3 (di-cerium tri-oxide), that can briefly store oxygen and will bind or release it as and when required.
A linear pre-converter lambda probe is arranged upstream of the catalytic converter so that mixing can be controlled. Said probe measures the residual oxygen component contained in the exhaust gas. A post-converter lambda probe downstream of the catalytic converter serves to monitor the catalytic converter function. The catalytic converter's oxygen storage capacity is therein checked using what is termed OSC-based catalytic-converter diagnosing (OSC=oxygen storage capacity). Rich/lean oscillating of the mixture is produced for this purpose through pre-controlling by the lambda controller. An intact catalytic converter will compensate oscillating using its oxygen storage capacity so that the post-converter lambda probe's probe voltage will oscillate with only a small amplitude. If, though, the catalytic converter has lost its oxygen storage capacity through ageing, the residual oxygen content will be similar upstream and downstream of the catalytic converter and the post-converter lambda probe's signal will oscillate widely.
The post-converter lambda probe is often used, moreover, to compensate long-term drifting in the pre-converter lambda probe's signal. This is referred to also as trimming.
The post-converter lambda probe's signal is therefore usually constant in the case of a catalytic converter having sufficient oxygen storage capacity and a properly functioning lambda controller. If the signal rises or falls, the catalytic converter has been either sated with oxygen or completely emptied of oxygen so that it will no longer be able to compensate a fluctuation in the fuel/air ratio. This is referred to also as “breaking through” of the post-converter lambda probe's signal to a rich or, as the case may be, lean mixture.
Breaking through of the post-converter lambda-probe signal hence indicates that the catalytic converter's oxygen storage capacity is exhausted or that no more oxygen is stored. However, no information is available between said two limiting values about the catalytic converter's actual, current oxygen loading. Said information would, though, be very helpful for maintaining the oxygen loading at around half the storage capacity and hence for providing the same buffering on the rich and lean side, as a result of which breaking through of the post-converter lambda-probe signal will be preventively avoided and the most favorable conditions for catalytic-converter diagnosing furthermore created.